An interesting study found that sniffing women's tears significantly reduced male aggression and reduced activity in brain networks associated with aggression. Research suggests this effect is caused by chemical signals in tears, a substance also seen in rodents that has protective functions.
Charles Darwin was puzzled by emotional tears; he thought they served no purpose other than to lubricate the eyes. Although tearing was thought to be a uniquely human trait, since Darwin, researchers have discovered that mammal tears contain chemicals that serve as social signals, one of which is reducing aggression.
For example, the tears of female mice contain signals that shut down aggression among male mice by suppressing activity in aggressive brain networks in male mice. Additionally, subordinate male blind mole rats smear themselves with tears to reduce aggression from dominant males.
Now, researchers at Israel's Weizmann Institute of Science have conducted a series of experiments to investigate whether sniffing human women's tears can reduce male aggression, as it does in rodents, and what functional effects this has on the male brain.
"We know that sniffing tears reduces testosterone, and that lowering testosterone has a greater impact on aggression in men than in women, so we looked at the effects of tears in men first because that made us more likely to see an effect," said Shani Agron, first author and co-corresponding author of the study.
Evidence on the chemical signature of human tears is limited, but a previous study by some of the researchers involved in this study found that women's tears contained an odorless chemical signature that, when sniffed, reduced self-rated sexual arousal, physiological arousal and testosterone levels.
First, the researchers tested whether sniffing women's tears would reduce men's aggression. The researchers collected "emotional" tears from six human donors aged 22 to 25 who individually watched sad movie clips to induce crying. Twenty-five men were asked to play a two-player money game against an opponent they were told was a human but was actually a computer algorithm. The purpose of the game is to elicit an aggressive response from men toward their opponent, who they will believe is cheating. If given the chance, men can retaliate against their opponents by making them lose money without benefiting from it themselves.
Before playing the game, participants sniffed the women's tears or saline (both are odorless), but they were not told what they were sniffing. The researchers observed a 43.7% reduction in aggression after exposure to tears. To assess the robustness of the findings, the researchers conducted a bootstrapping analysis, a statistical procedure that creates many simulated samples by resampling a single data set. The analysis found that the probability of achieving this result by chance was 2.9 percent, suggesting that chemical signals in human emotional tears have the same primary function of blocking aggressive behavior as in rodents.
Next, the researchers analyzed the effects of sniffing tears on the participants' brains. After being exposed to tears or saline, 26 male participants played a money game while undergoing functional magnetic resonance imaging (fMRI) scans. The researchers noticed that two brain structures associated with aggressive behavior - the left anterior insula cortex (AIC) and the bilateral prefrontal cortex (PFC) - showed reduced activity after exposure to tears. There was a clear correlation between experimental condition (tears vs. saline) and activity in these regions.
When studying the brain's functional connections, they found that tears only affected the left AIC, which had significantly increased connections with the right amygdala and piriform cortex. In addition to sharing structural connections, these regions are part of a functional network related to olfaction (smell) and aggressive behavior.
Noam Sobel, another corresponding author of the study, said: "We have shown that tears activate olfactory receptors, alter brain circuits related to aggression, and significantly reduce aggressive behavior. These findings suggest that tears act as a 'chemical blanket' that provides protection against aggressive behavior - an effect that is common in rodents and humans and may also apply to other mammals."
In fact, a 2022 study found that dogs shed significantly more tears when they were reunited with their owners rather than with familiar non-owners, suggesting they shed tears of happiness. However, further research is needed to determine whether these tears contain chemical signals that can be picked up by other dogs or humans.
After confirming the effects of tear-sniffing on male behavior, researchers hope to expand the scope of their study.
"When we looked for volunteers to donate tears, we found that the majority were women because crying is more socially acceptable for them," Agron said. "Now, however, we must extend this study to women to understand the impact more fully."
The research was published in the journal PLOS Biology.